It is known that lactacystin of the formula ##STR2## having neuritogenesis activity, is produced by culturing a physiologically active substance-producing microorganism sp. OM-6519 belonging to the genus Streptomyces, and isolating the thus-produced compound hereinabove (J. Antibiotics, 19:44, 113-116 and Japan, Pat. Unexam. Publ., No. 3-98594). However, this compound is highly cytotoxic and shows no selective toxicity.